In order to insure that networks ("nets") in a circuit board are not shorted together and do not have an unacceptable internet (leakage) resistance therebetween, the isolation material between one net and all of the other nets of the circuit board must be tested. Such a test, i.e., the socalled "shorts" test, typically has two criteria; first, the capability to detect an isolation resistance less than a specified threshold; and, second, the capability of voltage-stressing the isolation material between interconnects. It is also important that shorts are not destroyed by the voltage applied thereto during such stressing. In this regard, if a short is blown in an uncontrolled manner, the debris may remain inside the circuit board, and possibly redevelop into a short.
More specifically, voltage-stressing of the interconnect isolation material is necessary for the detection of latent defects. Such defects can consist of isolation material having characteristics that are voltage-dependent and/or time-dependent. An example of a latent defect is isolation material that initially passes a resistance test, but over time degrades in resistance to the point that it becomes a noticeable short. It has been found that stressing the isolation material during shorts detection can hasten the change of latent defects, thus revealing such defects during the test; whereas, a shorts measurement test lacking voltage-stressing would not cause alteration of the defective isolation material, and the defect would thus remain undetected.
As an example, consider a latent defect between nets consisting of a high leakage resistance that is voltage-dependent. Assume that short detection means is being used that, for this specific case, is capable of detecting a leakage resistance up to 10 Mohms, and that a stress voltage of 10 volts is applied across the defect and the resistance measured is large, i.e., over 10 Mohms. The net is therefore considered as good or acceptable. However, if a voltage-stress of 50 volts is then applied, the resistance measured would become small, i.e., less than 1 Kohm. The leakage resistance in this instance falls within the sensitivity of the short-detection means, and the net is thus flagged as defective.
The traditional testing method for packaging products, such as printed circuit boards and substrates, is the cluster probe, or "bed-of-nails" method. In the cluster-probe method of testing, all of the interconnects, or nets, in a packaging device are connected to the tester simultaneously, i.e., at least one probe is physically placed on each of the nets at the same time. Shorts between nets are detected by measuring the leakage current (and hence the resistance) between different nets. Ideally, this current would be very small. With this method of testing shorts, all nets but one are electrically connected. The isolation material between the one net and all of the linked nets is then tested using a resistive measurement. This method has the capability of voltage-stressing the isolation material between nets up to a voltage limit set by the test equipment and the device-under-test for as long a dwell time as desired. Cluster-type probe testing is advantageous in that very high throughput is achievable, and large volumes of products having the same pattern or footprint can be efficiently tested. The problem with the cluster-probe shorts test, therefore, is not in the measurement, but is in the fact that a unique probe head must be built to match every product. This is often cost prohibitive and requires long lead-times for designing and building the necessary equipment.
To avoid the high capital expense and long lead-time of the cluster-probe testing, an alternative method, called moving-probe testing, is used. Instead of having a fixed test probe that exactly matches the device to be tested, the moving-probe tester has probe mechanisms that can dynamically place a limited set of probes onto selected interconnects. Moving-probe testers are, therefore, able to test a wide diversity of devices, each with its own unique footprint, in a more cost-effective and timely fashion. There are tradeoffs, however, to the flexibility gained through the use of the moving-probe test method. If a moving-probe tester was to use the resistive method for shorts detection, a probe would have to be placed on the net to be tested, and another probe would have to be sequentially placed on each of the other nets. This would result in N(N-1)/2 measurements, where N is the number of nets. This test would be able to detect and locate shorts and apply a voltage-stress to the isolation material between nets, similar to the method used by the cluster-probe tester. The test would, however, become unacceptably slow as the number of nets in a device increases. Therefore, faster methods for short-detection and voltage-stressing with a moving-probe are desirable.
In order to improve the speed of the moving-probe shorts test, measurements other than a two-probe resistance test are used. These techniques make use of a single moving probe to contact the net to be tested and use a fixed probe to contact a reference plane during the measurement. The large number of measurements of the previous shorts test methods is thereby avoided since, ideally, the moving probe must contact each net only once, resulting in N total tests. Typically, the measurement used is capacitive, though this need not be the case. Conventionally, although this method of shorts detection reduces test time, it does so at the expense of measurement sensitivity and voltage-stressing capability. Prior art patents which discuss short detection using a moving probe, but addressing only the resistance criteria of shorts detection, and not discussing voltage-stressing of the net using a moving probe, include U.S. Pat. No. 4,565,966, issued Jan. 21, 1986 to Burr et al.; U.S. Pat. No. 5,006,808, issued Apr. 9, 1991 to Watts (the "Watts" patent); commonly assigned, U.S. Pat. No. 5,266,901, issued Nov. 30, 1993 to Woo (the "Woo" patent); and, commonly assigned, U.S. Ser. No. 843,672, filed on Feb. 28, 1992, Halperin et. al., pending ("Halperin et.al.").
It has been generally accepted that internet, or net-to-net, voltage-stressing cannot be accomplished with the moving probe test methodology because only one test point is contacted at a time.
Thus, there remains a need for voltage-stressing capability with single moving-probe shorts testing. Further, the voltage-stressing should terminate after an appropriate stress period for low-resistance shorts, so that such shorts do not burn out due to the voltage-stressing.